1. Field of the Invention
This invention relates generally to a DC to AC inverter system, and, more particularly, to an AC motor control system.
2. Description of the Prior Art
Variable speed motor drive circuits are well known and are used to provide easy and versatile control of powered drive systems. There are two basic types of speed control systems; one for DC or universal motors and the other for AC induction motors. A DC motor control system requires a minimum of power and control circuitry, but the DC motor is itself expensive to purchase and maintain. The electronic control system for an AC motor is more complex, but this is offset by the economic advantage of using an AC motor, the cost of which may range from one-half to one-fifth the price. Moreover, since an AC motor has no brushes or commutators, it requires less maintenance, is more rugged and reliable, and can operate in explosive, dusty, or highly humid atmospheres and at high altitudes. In addition, AC motors can have higher maximum speeds than DC motors and can be liquid cooled. These advantages, when coupled with the availability of standard rated motors, make AC motors more attractive for most industrial applications than DC motors.
The speed of an AC induction motor is controlled by varying the amplitude and/or frequency of its supply voltage. The variable voltage, fixed frequency technique has the disadvantage that the maximum torque of the motor is proportional to the square of the applied voltage; therefore, this control method is effective over only small torque ranges. Better speed control is obtained through the use of a variable frequency drive system, but certain motor characteristics may represent restraints which limit the overall effectiveness of this approach. Generally, the best speed control is obtained through a system which varies frequencies, voltage and the number of pulses per half cycle.
Regardless of what type of speed control apparatus is used, a motor must be operated within its nameplate ratings. This rating gives the safe torque and speed limits for the motor as determined by the temperature limit, maximum safe rotational speed and the saturation limit of the iron used in the motor. To keep from saturating the iron, the RMS stator voltage must be proportional to the frequency of this voltage. In this case, the maximum producible torque is independent of the frequency of the applied voltage. However, maximum torque occurs at a particular value of slip at a particular supply frequency. Slip is defined as (f.sub.stator -f.sub.rotor)/f.sub.stator where f.sub.stator is the stator frequency and f.sub.rotor is the rotor frequency. By properly controlling the slip, the operating characteristics can be optimized and power requirements for a given load minimized. Further, by controlling the slip, it is possible to achieve a high torque-to-current ratio.
Two types of circuits are used to provide variable frequency alternating voltages for motors. These are commonly referred to as inverters and cycloconverters.
A cycloconverter changes the frequency of a three phase alternating power source without the necessity of an intermediate AC to DC conversion stage. Cycloconverters require multiple control devices with three phase excitation, and are limited to low output frequencies. The output of a cycloconverter has a relatively large ripple voltage, but comparatively small ripple current which is what contributes to motor heating. The higher the output frequency, the greater the ripple current since there are fewer supply pulses per output cycle. This limits the practical upper frequency for a cycloconverter to one-sixth supply frequency if a half-wave converter is used, and to one-third the supply frequency if a full wave converter is used.
The preference for AC motors and the reasons therefore have been described above. However, in many cases only direct current power sources (e.g. batteries) are available. For example, the recent energy crisis has resulted in the pursuit of electric vehicles which are both reliable and efficient. It is therefore necessary that means be provided for controlling an AC motor which is powered by a DC source. This may be accomplished through the use of an inverter system which converts DC to AC power and which can be designed for variable frequency output.